On the Seismic Response of Anchoring Elements for Submerged Floating Tunnels

Submerged Floating Tunnels (SFTs) are interesting modular structures that lend themselves well to the crossing of water bodies. In a widespread design configuration the SFTs retain positive buoyancy and are kept at a fixed depth under the water surface by anchoring devices connected to the seabed. Different technological solutions have been proposed for the anchoring devices. They all share the properties of being slender, axially stiff and very flexible in transverse direction. The positive buoyancy of the SFT is typically tailored such to avoid detensioning (slackening) of the anchoring elements under design loading conditions, e.g. wave and earthquake loading. Whenever the tunnel is located in seismic prone areas, the seismic loading can be the dominant one. The present paper focuses on the local dynamic response of anchoring elements of a proposal for the Messina Strait crossing. The seismic input is modeled according to a suitable power spectral density of the ground acceleration and a reduced-order model of anchoring element is set up according to the classic theory of small-sag cables. The seismic working conditions are assessed with respect to slackening requirements.